Conflict between Local and Global Changes in Plant Diversity through Geological Time

Reliable measurement of the original diversity of paleovegetation must account for many influences: the amount of time and space represented by the sample, the number of plant parts in the sample, the stature of the paleovegetation in relation to sample area, and the preseruational quality of the sample, as well as the number of species in the source community and their relative abundances. Plant compression fossil assemblages from river or delta fioodplain environments generally have undergone minimal time and space averaging, which allows species richness to be measured and compared at a variety of temporal and spatial scales. Comparison of late Paleozoic and early Cenozoic wet fioodplain vegetation reveals little difference in species richness or diversity at the smallest measurable spatial scale, based on samples of a few square meters representing 0.1-0.5 hectares of source vegetation. Outcrop transects several hundred meters to several kilometers long that sample across minor variations in wet fioodplain habitat also show similar leuels of diversity in the Paleozoic and Cenozoic, with the exception of one Cenozoic transect with very high diversity. Diversity differences between Paleozoic and Cenozoic sites and transects are far less than would be expected based on global species richness curves for the Devonian through Pleistocene. The small differences in the diversity of wet fioodplain vegetation probably relate to several factors, including the number of trees that can coexist in a small area, difficult edaphic conditions on wet floodplains, and perhaps the geologically early colonization and saturation of these habitats. The difficulty of making reliable measures of plant diversity in the fossil record, and the absence of change observed with relatively high quality data, suggest that biological explanations for changes in global species richness through geological time are premature. INTRODUCTION